What is a good insulator at 120 volts but acts as a conductor at 10,000 volts?

Air is typically a good insulator under normal conditions, particularly at lower voltages such as 120 volts. However, when exposed to extremely high voltages, like 10,000 volts, air can become ionized, allowing it to conduct electricity. This phenomenon occurs because the electric field strength exceeds the breakdown voltage of air, which can cause it to become conductive, leading to phenomena such as arcing.

In contrast, materials like rubber and wood are generally good insulators at both low and high voltages, maintaining their insulating properties. Water, while able to conduct electricity, especially when impurities are present, is a more complex medium as it behaves differently depending on its purity and other factors. Thus, air's specific properties make it unique in this context, capable of transitioning from insulator to conductor under high voltage conditions.